The invention relates to a fuel system in particular of a motor vehicle with a fuel tank and a ventilation device for ventilating the fuel tank, wherein the ventilation device has a separation device which has at least one temporary accumulator for liquid fuel. The invention further relates to a method for operating a fuel system.
Fuel systems of the aforementioned type are known from the state of the art. They are assigned for example to a motor vehicle or a drive system of the motor vehicle. The drive system includes in particular at least one internal combustion engine and is for example configured as hybrid drive system, i.e. it includes the internal combustion engine as well as at least one electrical machine, wherein the internal combustion engine and the electrical machine at least temporarily concertedly generate a drive torque of the drive system. The internal combustion engine is supplied with fuel from the fuel tank by the fuel system. Oftentimes, a volatile hydrocarbon fuel, for example gasoline, is used as fuel. The fuel tank therefore normally contains a volume of liquid fuel as well as a volume of gaseous fuel, which accumulates in particular above the liquid fuel. The fuel tank can be a closed tank in particular a pressure tank, or a partially closed, in particular also pressure free tank. The closed tank is used in particular for reducing emissions.
Due to temperature fluctuations of the fuel, for example caused by changes of the ambient temperature, pressure fluctuations can occur in the fuel tank. For this reason, the ventilation device is assigned to the fuel tank. It serves for ventilating the fuel tank. In this way, excessive pressure in the fuel tank can be reduced by the ventilating device. For this purpose the ventilation device ventilates the fuel tank for example by a ventilation line. During ventilation, gaseous as well as liquid fuel can exit the fuel tank through the ventilation device or the ventilation line. The ventilated fuel thus is first present as a mixture of gaseous and liquid fuel. This is particularly the case when the fuel tank is ventilated at a high pressure inside the fuel tank. As a result of the high pressure or the great pressure difference between the pressure inside the fuel tank and the pressure outside of the fuel tank, the ventilated fuel has high flow velocities, which causes liquid fuel to be carried along by the gaseous fuel.
The gaseous fuel can readily be fed to the internal combustion engine or its intake system respectively, wherein a fuel accumulator which is preferably configured as activated carbon filter can be provided between the fuel tank and the internal combustion engine. The fuel accumulator serves for temporarily storing gaseous fuel, i.e. for taking up gaseous fuel when unused gaseous fuel is present and for giving off gaseous fuel as soon as the gaseous fuel can be discharged into the internal combustion engine. However, liquid fuel must not enter the fuel accumulator or the internal combustion engine.
For this reason, the ventilation device has the at least one separation device which serves for separating gaseous and liquid fuel. The separation device thus has the purpose to prevent the transfer of liquid fuel from the fuel tank through the ventilation device into the internal combustion engine. The separation device separates liquid fuel and allows gaseous fuel to pass. The separated liquid fuel enters the temporary accumulator of the separation device. The term temporary accumulator does not mean that a (temporary) accumulation of the liquid fuel is indeed provided for. Rather, the liquid fuel can be discharged directly from the temporary accumulator or the separation device preferably in the direction of the fuel tank. Here, however, a rise of the fill level of the temporary accumulator may result, for example due a limitation of the discharge volume flow, in particular by a line cross section or the like. The separated liquid fuel thus temporarily cannot be discharged at the same rate as it enters the temporary accumulator. Of course, a temporary storage of the liquid fuel, for example over a defined period of time can also be realized.
When operating the fuel system, the amount of liquid fuel present in the temporary accumulator or the separation device should be prevented from exceeding a threshold amount, i.e. from being greater than the fill level of the temporary accumulator, because this may impair the effectiveness of the separation device. The greater the amount of liquid fuel in the temporary accumulator, the greater is the risk that liquid fuel also escapes the separation device together with the gaseous fuel and is carried along in the direction of the fuel accumulator or the internal combustion engine. For this reason, the fuel delivery device can be assigned to the separation device. The former is used to deliver liquid fuel out of the temporary accumulator, in particular in the direction of the fuel tank. The fuel delivery device is usually configured as ejector pump wherein fuel which is delivered by a fuel pump of the fuel system out of the fuel tank in the direction of internal combustion engine, is often used as means for driving the ejector pump. In fuel systems known from the state of the art it may happen, however, that liquid fuel escapes the separation device again together with the gaseous fuel and is carried along in the direction of the fuel accumulator of the internal combustion engine.